Lampsacus was an ancient Greek city strategically located on the eastern side of the Hellespont in the northern Troad. The name has been transmitted in the nearby modern town of Lapseki.

Originally known as Pityusa or Pityussa, it was colonized from Phocaea and Miletus. During the 6th and 5th century BC, Lampsacus was successively dominated by Lydia, Persia, Athens, and Sparta; Artaxerxes I assigned it to Themistocles with the expectation that the city supply the Persian king with its famous wine.

Lampsacus joined the Delian League after the Battle of Mycale, and paid a tribute of twelve talents, a testimony to its wealth, and it had a gold coinage in the 3rd century BC, an activity only available to the more prosperous cities. A revolt against the Athenians in 411 BC was put down by force.

In 196 the Romans defended the town against Antiochus the Great, and it became an ally of Rome; Cicero (2 Verr. i. 24. 63) and Strabo (13. 1. 15) attest its continuing prosperity under Roman rule.

Lampsacus was also notable for its worship of Priapus, who was said to have been born there.